The Mustang came out first, but the first few years were lame IMO, it was a pussy car. i think Ford had much better choices for a performance car. In 67-69 when the Camaro came along both cars could be had with big blocks, and had multiple combos good for many different driving styles. Both cars competed at the drag strip, and Trans Am racing. Into the 70s both cars began to suck, but into the 80s and Ford got back on the stick before GM again with the foxbody. Flash ahead to any 4th gen f-body and there was a decent battle. Take away the Camaro again and Ford steps it up with the 03-04 Cobra. Now the Camaro is back and just as good.
This battle is pathetic, GM is always late to the party, and Ford won't try to kick GM's ass when they're actually making the car. Sales of the foxbodys was always better for a few reasons, for one they were geared towards the masses. F-body's were great for most enthusiests they were low and sleek, and always tended to perform better straight from the factory. However this always made them less practical, they sucked in bad weather, were harder to get into and out of, so people really had to like it. Mustang on the other hand didn't have much of these issues, and foxbodys could be had with a 4 cylinder most years making them "sporty" and economical. So yeah Mustang has been around longer and sold more cars, DUH! Being a die-hard GM guy and loving the mods and performance of the F-body over the Mustang, it's a clear choice for me.